Le Morte d'Arthur:1,2 (thing)

How Uther Pendragon made war on the duke of Cornwall, and how by
the mean of Merlin he lay by the duchess and gat Arthur.

THEN Ulfius was glad, and rode on more than a pace till that he
came to King Uther Pendragon, and told him he had met with
Merlin. Where is he? said the king. Sir, said Ulfius, he will
not dwell long. Therewithal Ulfius was ware where Merlin stood
at the porch of the pavilion's door. And then Merlin was bound
to come to the king. When King Uther saw him, he said he was
welcome. Sir, said Merlin, I know all your heart every deal; so
ye will be sworn unto me as ye be a true king anointed, to fulfil
my desire, ye shall have your desire. Then the king was sworn
upon the Four Evangelists. Sir, said Merlin, this is my desire:
the first night that ye shall lie by Igraine ye shall get a child
on her, and when that is born, that it shall be delivered to me
for to nourish there as I will have it; for it shall be your
worship, and the child's avail, as mickle as the child is worth.
I will well, said the king, as thou wilt have it. Now make you
ready, said Merlin, this night ye shall lie with Igraine in the
castle of Tintagil; and ye shall be like the duke her husband,
Ulfius shall be like Sir Brastias, a knight of the duke's, and I
will be like a knight that hight Sir Jordanus, a knight of the
duke's. But wait ye make not many questions with her nor her
men, but say ye are diseased, and so hie you to bed, and rise not
on the morn till I come to you, for the castle of Tintagil is but
ten miles hence; so this was done as they devised. But the duke
of Tintagil espied how the king rode from the siege of Terrabil,
and therefore that night he issued out of the castle at a postern
for to have distressed the king's host. And so, through his own
issue, the duke himself was slain or ever the king came at the
castle of Tintagil.

So after the death of the duke, King Uther lay with Igraine more
than three hours after his death, and begat on her that night
Arthur, and on day came Merlin to the king, and bade him make him
ready, and so he kissed the lady Igraine and departed in all
haste. But when the lady heard tell of the duke her husband, and
by all record he was dead or ever King Uther came to her, then
she marvelled who that might be that lay with her in likeness of
her lord; so she mourned privily and held her peace. Then all
the barons by one assent prayed the king of accord betwixt the
lady Igraine and him; the king gave them leave, for fain would he
have been accorded with her. So the king put all the trust in
Ulfius to entreat between them, so by the entreaty at the last
the king and she met together. Now will we do well, said Ulfius,
our king is a lusty knight and wifeless, and my lady Igraine is a
passing fair lady; it were great joy unto us all, an it might
please the king to make her his queen. Unto that they all well
accorded and moved it to the king. And anon, like a lusty
knight, he assented thereto with good will, and so in all haste
they were married in a morning with great mirth and joy.

And King Lot of Lothian and of Orkney then wedded Margawse that
was Gawaine's mother, and King Nentres of the land of Garlot
wedded Elaine. All this was done at the request of King Uther.
And the third sister Morgan le Fay was put to school in a
nunnery, and there she learned so much that she was a great clerk
of necromancy. And after she was wedded to King Uriens of the
land of Gore, that was Sir Ewain's le Blanchemain's father.